In recent years, a refrigeration cycle device has been attracting attentions that uses, as a refrigerant, carbon dioxide, which has zero ozonosphere rupture potential and a markedly small global warming potential as compared with those of chlorofluorocarbons. The critical temperature of the carbon dioxide refrigerant is as low as 31.06 degrees C. When a temperature higher than this temperature is used, the refrigerant at a high-pressure side (from the outlet of a compressor, to a radiator, and then to the inlet of a pressure-reducing device) of the refrigeration cycle device becomes a supercritical state in which the refrigerant is not condensed, thereby decreasing operating efficiency (coefficient of performance, COP) of the refrigeration cycle device as compared with a conventional refrigerant. Hence, means for increasing COP is important for the refrigeration cycle device using the carbon dioxide refrigerant.
As such means, there is suggested a refrigeration cycle including an expander instead of the pressure-reducing device and recovering pressure energy during expansion to use the pressure energy as power. Meanwhile, in a refrigeration cycle device with a configuration in which positive-volume compressor and expander are coupled with one shaft, when VC is a stroke volume of the compressor and VE is a stroke volume of the expander, a ratio of circulation volumes of the refrigerants respectively flowing through the compressor and the expander is determined by VC/VE (a design volume ratio). When DC is a density of the refrigerant at the outlet of an evaporator (the refrigerant which flows into the compressor) and DE is a density of the refrigerant at the outlet of the radiator (the refrigerant which flows into the expander), a relationship of “VC×DC=VE×DE,” that is, a relationship of “VC/VE=DE/DC” is established since the circulation volumes of the refrigerant flows respectively flowing through the compressor and the expander are equivalent. VC/VE (the design volume ratio) is a constant that is determined when the device is designed. The refrigeration cycle tends to keep balance so that DE/DC (the density ratio) is always constant. (Hereinafter, the phenomenon is called “constraint of constant density ratio.”)
However, use conditions of the refrigeration cycle device may not be constant, and hence if the design volume ratio expected at the time of the design differs from the density ratio in the actual operating state, it is difficult to adjust the high-pressure-side pressure to an optimal pressure due to the “constraint of constant density ratio.”
Owing to this, there is suggested a configuration and a control method for adjusting the high-pressure-side pressure to the optimal pressure by providing a bypass passage that bypasses the expander and controlling the amount of refrigerant which flows into the expander (for example, see Patent Literature 1).
Also, there is suggested a configuration and a control method for adjusting the high-pressure-side pressure to the optimal pressure by providing a compression bypass passage that bypasses a phase from an intermediate position of a compression process of a main compressor to completion of the compression process and a sub-compressor provided in the compression bypass passage, and controlling the amount of refrigerant which flows into the sub-compressor (for example, see Patent Literature 2).